U.S. Pat. No. 5,028,858 discloses an electric power tool that is powered by two battery packs. In this electric power tool, two battery packs are connected in series, and a motor is driven with a high voltage. Due to this, the electric power tool can provide higher power that can be used in heavy work than a case where one battery pack is used as a power supply.
When battery packs are connected in series, large load may be applied to one battery pack. For example, if a charged level is different between two battery packs, one battery pack may be overcharged and may be charged in a reverse direction (i.e. reverse-charged) by the other battery pack. In this case, one battery pack may be damaged severely so that deterioration thereof progresses further or the battery pack may become unusable.
With regard to the above problem, in the above-described electric power tool, a charged level of each of two battery packs is measured, and when the charged level of the battery pack is out of an allowable range, a power supply circuit that supplies power from the battery pack to a motor is electrically disconnected. According to this configuration, it is possible to prevent overdischarging of the battery pack and prevent severe damage to the battery pack. Such a configuration is not limited to an electric power tool in which a plurality of battery packs is connected in series but can be effectively applied to an electric power tool in which a plurality of battery packs is connected in parallel and an electric power tool in which only one battery pack is used.